The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Airplane turbojet engines generate significant noise pollution. There is a high demand to reduce that pollution, particularly inasmuch as the turbojet engines used are becoming increasingly powerful. The design of the nacelle surrounding the turbojet engine contributes in large part to reducing that noise pollution.
In order to further improve the acoustic performance of aircrafts, nacelles are equipped with acoustic panels aiming to attenuate the noises due to the circulation of flows of air through the turbojet engine as well as the vibrations of the structures of the nacelle. Thus, acoustic panels are generally arranged along a circulation tunnel for the flows of air generated by the turbojet engine, from the air inlet upstream to its downstream outlet.
Acoustic panels are sandwich-type structures well known for absorbing these noises. These panels typically include one or more layers of cellular core structures (commonly called “honeycomb” structures). These layers can then be coated on their so-called external surface, i.e. the surface furthest from the aerodynamic flow, with an air-impermeable skin, said to be “solid,” and on the internal surface, i.e. the surface closest to the aerodynamic flow, with an air-permeable perforated skin, said to be “acoustic.”
In a known manner, the cellular core structure is made from attached cellular units having cells that are generally hexagonal or elliptical.
Other structures, designed in particular to meet specific acoustic criteria, may be used. Examples in particular include linear degree of freedom (LDOF) panels, where a fine metal grate is glued on the acoustic skin, and so-called double degree of freedom (2DOF) panels comprising two levels of cellular cores separated by a porous septum.
In general, the present disclosure is not limited to a particular acoustic panel structure and will be compatible with these various structures.
One of the main drawbacks is the fastening of these panels. In fact, these panels are generally fastened by screws that can be disassembled so as to allow and facilitate maintenance or replacement operations.
These screws may either pass through the panel at the honeycomb core, which is then locally reinforced, in particular by the addition of resin, a solid reinforcement, or an insert, or pass through the panel at a so-called skin return area where the honeycomb is locally eliminated.
In all cases, it will be understood that the acoustic surface of the panel is reduced as a result.
Another possibility consists of gluing the panel or riveting it on its solid external skin, as for example described in document EP 1,020,845, but in that case, the panels cannot be disassembled. Furthermore, in the case of document EP 1,020,845, it will be noted that the rivets still penetrate the inside of the panel, which still reduces the effective acoustic surface.
Document U.S. 2009/0242321 proposes one such solution by providing lateral fastening edges. It should nevertheless be noted that the presence of these fastening edges lengthens the panel, which can be problematic in areas with a defined and precisely delimited space. In that case, with an identical length, there will be a loss of acoustic surface.